1. Field of the Invention
The present invention relates to a communication terminal and a method for use in a radio communication system.
2. Description of the Related Art
Recent rapid popularization of the Internet, manifestation of pluralistic information, higher capacity and development of the next generation Internet have been promoting research and development of the next generation radio access schemes for implementing fast radio transmission in mobile communications. In such a fast radio environment, it is important to make not only downlink transmissions but also uplink transmissions faster. The 3rd Generation Partnership Project (3GPP) proposes an Enhanced Uplink scheme as an uplink speeding-up technique.
FIG. 1 illustrates a conventional W-CDMA (Wideband-Code Division Multiple Access) based communication system. In uplink communications in accordance with the W-CDMA scheme, data is transmitted in a radio transmission channel called a DPDCH (Dedicated Physical Data CHannel). A base station controls uplink transmission power depending on reception quality of the DPDCHs for communication terminals. For example, the reception quality may be represented by an indicator such as a SIR (Signal-to-Interference Ratio) and a bit error ratio. The base station uses a downlink transmission power control signal called a transmission power control (TPC) bit to control transmission power of the communication terminals within a relatively narrow range such as ±0.5-2 dB. See 3GPP TS25.214 V6.9.0 for this type of technique, for example.
On the other hand, in the Enhanced Uplink scheme, a communication terminal transmits data in a radio transmission channel called an E-DPDCH (Enhanced-Dedicated Physical Data Channel). A base station measures interference of the E-DPDCHs from the communication terminals and determines the allowable maximum of uplink transmission power for the individual communication terminals depending on the measured interference. The allowable maximum of uplink transmission power is derived from scheduling grants generated by the base station. The base station determines the scheduling grant for each of the communication terminals and transmits the scheduling grants to the communication terminals. The communication terminals may transmit uplink signals at the allowable maximum power designated in the scheduling grant or at a power level below the allowable maximum power. The communication terminals make the determination as to which power level is applied to the uplink transmissions. This differs from the W-CDMA based system in that the uplink transmission power is controlled through TPC bits transmitted to the communication terminals in accordance with the W-CDMA system. See 3GPP TS25.331 V6.9.0 for detailed calculation of the allowable maximum power level in the Enhance Uplink scheme, for example.
FIG. 2 schematically illustrates that the uplink transmission power varies over time. As illustrated, the uplink transmission power relatively slowly varies over time in the W-CDMA scheme. On the other hand, the uplink transmission power relatively drastically varies over time in the Enhanced Uplink scheme. The Enhanced Uplink scheme enables a communication terminal to determine the uplink transmission power flexibly. As a result, the uplink transmission power can be controlled more dynamically, resulting in effective utilization and power resources and throughput improvement. Instead, the communication terminal is required to support a wider dynamic range of the uplink transmission power than conventional schemes.
Meanwhile, there are some cases where base stations use different frequencies to accommodate a larger number of users. If a communication terminal presently communicating in a certain frequency migrates to a base station having a different frequency, the communication terminal must perform cell search for the different frequency. In general, the communication terminal has only one frequency oscillator. For this reason, in the cell search for the different frequency, the communication terminal must stop the communications, switch to the different frequency for the cell search and switch back to the original frequency. To this end, a certain time period is predefined for the different frequency measurement in such a system environment so that the communication terminal can measure communication environments for the different frequency. This time period is called a transmission gap. In an operating mode called a compressed mode, the different frequency measurement occurs at a frequency while using the transmission gap. See 3GPP TS25.214 V6.9.0, for example.
FIG. 3 schematically illustrates that the compressed mode is applied to the transmission power control as illustrated in FIG. 2. The communication terminal switches the presently synchronized frequency to a different frequency within the transmission gap and accordingly does not transmit any data during the transmission gap. As a result, the uplink transmission power falls to zero in the transmission gap and significantly varies immediately after the start and the end of the transmission gap.
As stated above, the uplink transmission power may be lower in the W-CDMA scheme than in the Enhanced Uplink scheme. For this reason, the W-CDMA scheme may cause less significant power variations immediately after the start and the end of the transmission gap. Thus, the communication terminal can relatively smoothly stop and restart transmissions.
On the other hand, the Enhanced Uplink scheme may cause highly significant power variations immediately after the start and the end of the transmission gap, as illustrated in FIG. 4, resulting in some problems in the stop and restart of transmissions. For example, a power amplifier (RF amplifier) in the communication terminal cannot follow the drastic power variations, and distorted signals may be transmitted due to inappropriate power. Also, a base station serving as a receiver of uplink signals also fails to follow the drastic power variations of the received signals, which may degrade reception quality such as causing increased bit errors or missing the received signals.